1. Field of the Invention
The present invention relates to a method for increasing the stability of hydroxyapatite coatings, or other calcium phosphate coatings, on metal implants. The coated metal substrates have important applications in dental and orthopaedic areas as implant materials.
2. Description of the Prior Art
Hydroxyapatite and tricalcium phosphate ceramics have been used as hard tissue replacement materials particularly in the dental areas for many years. The major inorganic component of human bones and teeth is biological apatite. Because of the similarity in chemical composition and crystal structure between hydroxyapatite ceramic and biological apatite, hydroxyapatite ceramic has excellent biocompatibility in comparison with other implant materials such as metals and plastics. Besides the excellent biocompatibility, hydroxyapatite ceramic also has conductive effect for bone growth and can bond very strongly with bone. Most of the clinical studies indicate that dense hydroxyapatite ceramics are not bioresorbable while other calcium phosphate ceramics are bioresorbable.
Currently, a coating of hydroxyapatite on metal substrates has expanded the medical applications of hydroxyapatite ceramic considerably. The coating of hydroxyapatite on the surface of metal implants provides excellent biocompatibility and can bond tightly with bone. The healing time of hydroxyapatite coated metal implants is much shorter than the un-coated metal implants. Currently, the hydroxyapatite coated metal implants have been used in the orthopaedic and dental areas. Many coating techniques can be utilized to coat hydroxyapatite or other calcium phosphate containing material on metal substrates. These include plasma flame coating, ion sputtering coating and high temperature compressing and sintering. At the present time, plasma flame spraying seems to be the most feasible technique for hydroxyapatite coating. However, due to the high temperature involved, the plasma flame coated hydroxyapatite is not pure hydroxyapatite. It always contains some decomposed products such as calcium oxide, amorphous hydroxyapatite, tricalcium phosphate and tetracalcium phosphate. The degree of decomposition depends strongly on the raw material used and the coating parameters. The coating of ceramics by the plasma flame method typically ends up with a porous structure. The decomposed products of hydroxyapatite normally have higher solubility values and dissolve readily in comparison with pure hydroxyapatite. Due to the decomposed products and the porous nature of hydroxyapatite by the plasma flame spraying technique, the bonding strength between the ceramics and metal substrate deteriorates in an aqueous environment such as the body fluid condition. Similar deterioration in coating bonding strength occurs for other calcium phosphate coatings. This raises concerns on the long term stability of the plasma flame coated hydroxyapatite or other calcium phosphate material. On the other hand, the ion sputtering coating can produce very dense coating. However, the coating thickness is very thin and most of the hydroxyapatite coating is either amphorous phase or un-identified calcium phosphates. This coating can be completely dissolved before bone growth.
A prior art technique discloses the post-treatment of plasma flame coated hydroxyapatite by removing the decomposed product, calcium oxide, to improve the biocompatibility of the plasma flame coated hydroxyapatite. By using special reagents the decomposed product calcium oxide is bleached out, the major calcium phosphate ceramic not being affected. However, this treatment does not improve the dissolution resistance of the coating nor reduce the deterioration rate of the coating strength of the hydroxyapatite coating in an aqueous environment.
What is thus desired is to provide a technique for decreasing the dissolution rate of the hydroxyapatite coating or other calcium phosphate based coating material on a metal substrate and to enhance the stability of the coatings in an aqueous environment.